This invention relates generally to diesel engines and, more particularly, to medium speed diesel engines operated with retarded fuel injection timing relative to conventional fuel injection timing.
In a diesel engine, liquid fuel is injected into a plurality of engine cylinders full of compressed air at high temperature. The fuel is broken up into droplets, which evaporate and mix with the air in the cylinders to form a flammable mixture. Traditional concerns for diesel engine manufacturers include fuel efficiency and smokeless operation, both of which relate to the quality of combustion in the cylinders.
The generation of some engine exhaust emissions, including carbon monoxide (CO), particulate matters (PM), and smoke, is related to a large extent to the ending time of the fuel injection duration during each piston cycle. Generally speaking, the earlier the fuel injection duration ending time, the less smoke and PM are generated in the combustion cycle. In some applications of medium speed diesel engines, such as for diesel locomotive operation, fuel injection timing, i.e., the start of the fuel injection duration, is set early in the cycle for full load smokeless operation. By applying early fuel injection starting time, the fuel injection duration ending time also occurs earlier in the piston cycle, which reduces PM, CO and smoke emissions. However, early fuel injection timing increases engine cycle temperature. Consequently, exhaust emission of nitrogen oxides (NO.sub.x) is relatively high.
One of the most effective ways to reduce emissions of NO.sub.x, a major concern to diesel engine manufacturers, is to retard fuel injection timing relative to conventional fuel injection timing, which results in a later fuel injection duration ending time, which, in turn, causes incomplete and untimely combustion in the cylinders, reduces engine efficiency, and increases generation of CO, PM, and smoke. Moreover, retarded fuel injection timing lowers the firing pressure in the cylinders, which under-utilizes the structural capability of the engine.
Decreasing the fuel injection duration time, which results in an earlier fuel injection duration ending time, can alleviate CO, PM and smoke emissions despite retarded fuel injection timing. However, increased fuel consumption will still result due to low cycle efficiency associated with untimely combustion from the retarded fuel injection timing.
Accordingly, it would be desirable to provide a diesel engine for operation at retarded fuel injection timing to reduce the generation of NO.sub.x without compromising engine efficiency and without incurring reduced cylinder firing pressures, therefore more fully utilizing the structural capability of the engine, and curbing the generation of CO, PM and smoke emissions.